Automatic oscillator frequency control system



C. TRAVIS April ll, 1939.

AUTOMATIC OSCILLATOR FREQUENCY CONTROL SYSTEM Original Filed Feb. 4, 1935 4 Sheets-Sheet 1 ci TRAVIS April 1.1, 1939.

AUTOMATIC OSCILLATOR FREQUENCY CONTROL SYSTEM 4 Sheets-Sheet 2 Original Filed Feb. 4, 1935 P P. .j H

INVENTOR CHARLES TRAVIS BY j, 4074A/ ATTORNEY MWF MGFQ mi Qms@ 35% C. TRAVIS April 11, 1939.

AUTOMATIC OSGILLATOR FREQUENCY CONTROL SYSTEM 4 Sheets-Sheet Original Filed Feb. 4, 1935 C. TRAVIS AUTOMATIC OSCILLATOR FREQUENCY CONTROL SYSTEM Original Filed Feb. 4, 1955 4 Sheets-Sheet 4 INVENTOR CWS TRAVIS ArroRNEY Q HUH April ll, 1939.

CTI

Patented Apr. 11, 1939 UNITED STATES AUTOMATIC osoILLATOR FREQUENCY CONTROL SYSTEM Charles Travis, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Original application February 4, 1935', Serial No. 4,793. Divided and this application December 12, 1936, Serial No. 115,562

9 Claims.

My present invention relates to superheterodyne receivers, and more particularly to improved methods of, and means for, automatically stabilizing the frequency of the local oscillator of a receiver of the superheterodyne type. This application is a division of application Serial No. 4,793, filed February 4, 1935.

The modern superheterodyne receiver is often used under conditions such that the local oscillator frequency, at any setting of the manually operated tuner, tends to shift. Local oscillator frequency shift may be due to thermal changes as well as line voltage fluctuations. It is inherent in the superheterodyne principle that the acf curacy with which tuning must be performed and maintained is directly proportional to the signal frequency. Present day short wave superheterodyne receiver design, for example, is not entirely adequate in regard to this accuracy. To keep the intermediate frequency within the middle ten percent of a 10 k. c. pass band is not particularly diicult at broadcast frequencies (550 to 1500 k. c.). However, at megacycles it requires an oscillator stability of one part in 40,000. This is equivalent to fair crystal control Without temperature compensation. At 20 megacycles local oscillator variations due to thermal effects and line voltage fluctuations amount to many kilocycles; in fact, frequency drifts of 40 or 50 k. c., are too often encountered.

Automatic frequency control (AFC) has been proposed in the past to regulate the local oscillator frequency. The utility of such a system in connection with a short wave receiver is readily appreciated when it is pointed out that even a moderately successful AFC system in such a case would greatly reduce the potential frequency drift. A ratio of to 1 would keep the signal carrier well within the I. F. pass band for a potential drift of as much as 50 k. c. It will be recognized that the desired station would not be entirely lost even though quality would suffer due to lack of alignment with the carrier.

For superheterodyne receivers used in the broadcast band the AFC system has many uses. Fine manual tuning required in high fidelity receivers to obtain best results could be avoided with such a system. The AFC, in general, may be said to be highly desirable for use in a superheterodyne receiver employed in a manner such that accurate tuning is important. In connection with remote tuning control systems an AFC system will be found valuable in providing the final adjustment that the remote control system is unable to effect.

':,Now, I have found, after investigation and experimentation, that an AFC system for a superheterodyne receiver should comprise a device, having frequency discrimination, which is capable of producing a control bias dependent upon the frequency of the intermediate frequency carrier, and an electronic local oscillator frequency control network, operated by the control bias, for adjusting the local oscillator frequency in a sense such that lit differs from a desired signal frequency by the established intermediate frequency.

It may, therefore, be stated that it is one of the main objects of the present invention to provide an automatic frequency control for a local oscillator, wherein the control includes a discriminator network comprising a pair of differentially connected rectiers which are mistuned by equal ramounts above and below the midchannel frequency of the intermediate frequency band; the discriminator functioning to provide a direct current-bias voltage which is dependent upon the mid-channel frequency, and the bias voltage being utilized tovary an electrical characteristic of an electron discharge device in. a manner such that the local oscillator frequency is adjusted to a desired value.

-Still another` object of the invention is to improve automatic frequency control systems for superheterodyne receivers, the improvement comprising vthe reduction of the intermediate frequency to .a lower frequency, the derivation-from the lower intermediate frequency energy of a control bias dependent in magnitude upon the lower frequency, and the employment of the control bias to regulate the magnitude of an electronic reactance in a sense to adjust the local oscillator lfrequency to a desired value.

And still other objects of the invention are to improve generally the efficiency of tuning of superheterodyne receivers, whether of the broadcast or short wave type, and more especially to provide automatic local oscillator frequency control systems for such receivers which are not only durable and reliable in operation, but readily manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in kconnection With the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be oarried into effect.

In the drawings:

Fig. 1 shows one embodiment of the invention,

Fig. 2 shows a modification of the local oscillator frequency control device used in Fig. 1,

Fig. 3 shows a modified form of an AFC system,

Fig. 4 schematically represents an AFC system applied to a superheterodyne receiver utilizing triple detection, and

Fig. 5 shows the system of Fig. 4 when employing the discriminator of Fig. 4 and the control device of Fig. 2.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 a circuit diagram of a. superhetercdyne receiver employing an AFC system. The receiver is of a conventional type well known to those skilled in the art, and comprises a signal collector A, such `as a grounded antenna circuit, coupled to a radio frequency amplifier having a variable tuning condenser device I in its input network. The amplifier may consist of more than one stage, and in such case the input of each stage would include a variable tuning condenser. The amplifier output is coupled to the tunable signal input circuit of the converter network, the input circuit including the variable tuning condenser 2.

The converter network comprisesa tube3known in the art as a pentagrid converter tube, also designated as a 2A'7 type tube (or 6A? when used for automobile receivers) the tube is designed to perform simultaneously the functions of a first detector and local oscillator. Since the particular construction of tube 3, and its circuits, is not a part of this invention, only a brief description will be given of the circuits thereof. The tunable signal circuit of tube 3 is connected between the signal grid and the grounded cathode. The local oscillator section of the network comprises the local oscillator tank circuit 4 connected between the grounded cathode and the rst grid functioning as the local oscillator grid.

The second grid of tube 3 is magnetically coupled to the coil 5 of the tank circuit 4, and the second grid therefore functions as the oscillator anode. The third and fifth grids act as screen grids for the fourth, or signal, grid. The resonant circuit 6 disposed in the plate circuit of tube 3 is tuned to the operating intermediate frequency. The tank circuit 4 includes in addition to the variable tuning condenser 1, the padding condensers 8, 9 which function to keep the tuning of the tank circuit 4 tracked with that of the signal circuits.

The rotors of condensers I, 2, I are arranged for mechanical uni-control, and the dotted lines I 0 represent such control. A manual tuner knob II is used to adjust the rotors from the operating panel of the receiver. It is suiiicient for the purposes of this application to explain that the condenser 'I tunes the tank circuit 4 through an oscillator frequency range differing at all times from the common signal frequency range of the amplifier and converter input circuits by the frequency of the circuit 6. For illustrative purposes the operating I. F. is assumed to be 450 k. c. It is to be clearly understood, however, that in place of the composite converter network there may be employed independent rst detector and local oscillator tubes in a manner well known in the art. Further, the signal frequency range may be the broadcast range, or it may be a short wave band.

The intermediate frequency energy in circuit 6 is transmitted to the diode second detector circuit of multiple duty tube I2 through a network including an I. F. amplifier. The latter may include one, or more, amplification stages, and is provided with an input transformer M1 having its primary and secondary circuits each resonated to the intermediate frequency; the output transformer M2 is similarly designed. Both transformers are designed to pass substantially uniformly a band of frequencies including the carrier and its modulation side band frequencies. The tuned secondary circuit I3 of transformer M2 is the input circuit of the diode second detector network.

The tube I2 is of the duplex diode-triode type, also well known in the art as a tube of the 55 or 2A6 type; although it is to be understood that a pentode section may be used in place of the triode section of tube I2 (2B'7 and 85 types). The diode anodes of tube I 2 are strapped together, and connected to the cathode side of grounded bias resistor I4 through a path including circuit I3 and load resistor I5, the latter being shunted by a radio frequency by-pass condenser I 6. The triode section of tube I2 has its control grid coupled to a desired point on load resistor I5 for deriving therefrom the audio component of the rectied intermediate frequency current. The amplied audio component in the plate circuit of tube I2 is then utilized in any desired manner.

'Ihe direct current component of the voltage developed across resistor I5 is used for AVC purposes. This may be accomplished in any manner well known in the art. By way of illustration the lead I'I, designated as AVC, is shown connected from the diode anode side of resistor I5 to the signal grid circuits of the radio frequency amplifier, the converter, and the intermediate frequency amplifier. The usual pulsating current filter elements I8, comprising a resistor and condenser, are disposed in the AVC connection. Those skilled in the art are fully aware of the mode of operation of the AVC system. As the received signal amplitude increases, the signal grids of the controlled tubes are biased increasingly negative thereby reducing the gain of these controlled tubes. In this way the signal input to the second detector is maintained substantially constant over a wide range of signal amplitude variation at the antenna A.

It is to be understood that the controlled tubes can be progressively decreased in AVC bias. This is the method of AVC biasing employed at the present time; a tube receiving less signal energy thus has applied to it stronger AVC bias. Again, the AVC bias may be derived from a separate diode rectifier; or from a point preceding the detector input circuit I3 whereby there will be less selectivity to the AVC rectifier than to the detector. These arrangements are all well known in the art. The specific type of AVC circuit employed in the receiver is immaterial; the essential thing is that such a system coact with the AFC system hereinafter described to produce satisfactory local oscillator frequency control.

To secure automatic control of the frequency tank circuit 4 there is employed a network comprising a frequency changer adapted to reduce the 450 k. c. I. F. to an I. F. of 50 k. c. The lower I. F. is then impressed upon a frequency discriminator which functions to pro- III duce a control bias dependent in magnitude upon the lower I. F. The control bias is used `to regulate the effect of an electronic device upon the tuning of the tank circuit 4.

Considering, now, thev AFC system from a specic viewpoint, the converter I9 is of the same type as that shown in connection Awith tube 3. The I. F. energy is impressed upon the signal grid through `coupling condenser 20, and the local oscillation network (not shown) is tuned to a frequency of 500 k. c. Thus, there is produced in the plate circuit of tube I9 a reduced I. F. of 50 k. c. pable of constructing the circuit details of network I 9 from the disclosed circuits associated with tube 3. The plate circuit of network I9 is coupled to the frequency discriminator network, the latter including an electron discharge tube of the duo-diode type.

The tube 2i may be of the type shown in connection with the tube I2; in that case the lgrid and plate will be left free and only the diode anodes 22 and 22' are used. The plate circuit of tube I9 includes a pair of coils 2'3, 24 arranged in series. Coil 23 is coupled to resonant circuit 23'; the latter is tuned to one side of the I. F. of 50 k. c. by a predetermined frequency amount. Thus, circuit 23 is turned to 49 k. c. Coil 24 is coupled to resonant circuit 2d', the latter being tuned to the other side of the I. F., as for example to 51 k. c. The diode anode 22 is connected to the cathode of tube 2! through a series path including circuit 23' and resistor 25, the latter being shunted by a by-pass condenser, and the low .alternating current side of circuit 23' being grounded.

The diode anode 2'2' is connected to the cathode of tube 2| through a series path including e. circuit 24 and resistor 26, the latter being shunted by a by-pass condenser. The anode side of resistor 26 has connected to it a lead El', designated as AFC; the lead comprising the path through which the control bias developed in the discriminator is applied to the electronic frequency control device. The lead 2? includes a resistor-c0ndenser network 28 which functions to suppress pulsating components in the control voltage transmitted through lead 2. A grounded switch 29 is connected to lead 2l, and its function will be explained in detail at a later point.

The device which func-tions to change the tuning of the tank circuit 4 comprises an electronic reactance element. It includes an electron discharge tube 30, which may be, for example, of the screen grid type. A bleeder resistor 3|, grounded at one end and connected at the other end to the +B side of the receiver voltage supply source, furnishes the requisite operating potentials of the electrodes of tubes 30. The lead 21 is connected to the control grid of tube 30 through a resistor 32', and a condenser 33 of a predetermined magnitude is connected between the control grid and anode of the tube. The control grid of the latter is also connected to the oscillator grid side of tank circuit 4 through a blocking condenser 34.

The operation of the AFC system lshown in Fig. l, as well as the characteristics of the component units thereof, will now vbe explained. In general, the operation of the AFC system is dependent on a change in I. F. .An undesired change of local oscillator frequency changes the response of the discriminator. This, in turn, causes a variation in the control bias fed .to the Those skilled in the art will be cagrid of tube 30.' There results, as a consequence, a change in the reactance of the tank circuit 4 in a direction such as to restore the tank circuit frequency to the desired value. The essential elements of the system are the discrim inator and the reactance lcontrol device.

Considering the characteristics of the discrimlnator, it can be stated that the latter should be constructed so that it will not be affected by adjacent channel signals. For -this reason the frequency discriminator must be preceded by a certain amount of I. F. selectivity, and the input-for' the discriminator is therefore taken ofi somewhere at, or near, the end of the I. F. amplifier. The system shown in Fig. l utilizes a second frequency changer network to reduce the 450 k. c. I. F. to 50 k. c.\ This second I. F. of 50 k. c. is employed to get greater selectivity in the frequency discrimination device. The frequency discriminator itself is shown as cornprising a pair of differentially connected diode rectifiers, the rectiers being mistuned by equal amounts above and below the mid-channel frequency.

By utilizing the differentially connected rectiers in the frequency discriminator network the output of the network depends solely on frequency of the carrier. A change in carrier amplitude in that case only affects the sensitivity of the control action. In spite of -this a satisfactory AVC system is desirable. As stated before the diode rectiers of the discriminator are connected differentially. That is to say, the difference of the rectifier outputs is utilized as the control bias to be transmitted through lead 21. Instead of employing diode rectifiers, it is possible to utilize in the balanced detector arrangement a pair of plate circuit detectors arranged in push-pull. However, it is more desirable to employ diode rectiers, because such an arrangement gives the difference in output directly.

The point on resistor 26 to which lead 2l is connected is at a negative direct current potential with respect to ground when the input from circuit 24 is greater than that from circuit 23', and it will be positive with respect to ground in the reverse case. When the I. F. is exactly between the resonant frequencies of circuits 23' and 24'., the inputs to the two rectifiers balance, and the point on resistor 26 to which lead 2 is connected will, therefore, be at ground potential. Accordingly it will be seen that the magnitude and polarity, with respect to ground, of the voltage of point Z on resistor 26 is wholly dependent upon the frequency value of the intermediate frequency energy produced in the plate circuit of tube I9.

The device which changes the frequency of the tank circuit 4 is shown in the system of Fig. 1 as being of the type comprising an electron discharge tube whose control grid to cathode capacity is utilized as a tuning condenser across tank circuit 4. Without entering into any eX- tended and detailed discussion of the electrical characteristics of tube 30 and its associated circuits, it is pointed out that the effect of the condenser 33 connected between the control grid and anode of tube 30 is to augment the capacitative magnitude of the control grid to cathode capacity of tube 30. It has been shown that in such a network, variation of the bias of the control grid results in a variation in the magnitude of the control grid to cathode capacity.

Reference is made to application Serial No.

638,514 of Jacob Yolles, led October 19, 1932, patented Oct. 15, 1935 as U. S. P. 2,017,270 for a disclosure of a voltage operated electronic condenser device which may be utilized, with suitable change of circuit constants for the purpose of the present invention. The magnitude of condenser 33 is chosen so as to furnish a control grid to cathode capacity suitable for adjustment of the frequency of tank circuit 4. It will be seen that this control grid to cathode capacity is connected in series with condenser 34, and both capacities are connected in shunt across the variable tuning condenser 1. Hence, by varying the bias of the control grid of tube 30 with respect to ground, there is secured a variation in the magnitude of the capacitative reactance which has been described as in shunt with tuning condenser 1. As the bias of the control grid of tube 30 is made negative, the gain of tube 30 is reduced, and the control grid to cathode capacity magnitude is diminished. This results because the magnitude of this capacity is dependent upon the gain of tube 30.

It will now be seen that a change in the frequency of the intermediate frequency energy, flowing in the plate circuit of tube I9, away from the lower I. F. frequency of 50 k. e. results in a simultaneous shift of the frequency of tank circuit 4. Of course, it will be understood that the change in the control grid to cathode capacity magnitude of tube 3U is to be in that direction which will cause the tank circuit frequency to be restored to its desired value. Those skilled in the art will be able to choose suitable constants for the circuits of the AFC system disclosed in Fig, 1 to suit the particular signal and oscillator frequency ranges employed.

Merely by way of example, let it be assumed that the signal frequency to which the set is tuned is 1500 k. c. In that case the frequency of the tank circuit 4 which is required in the production of the 450 k. c. I. F. is 1950 k. c. If, for some reason or other, the frequency of tank circuit 4 changes to 1951 k. c., there will be pro# duced in the output circuit of the I. F. amplifier energy of 451 k. c. Since the second frequency changer is operating with a local oscillator frequency of 500 k. c., it follows that the second intermediate frequency energy, produced in the plate circuit of tube I9, will have a frequency of 49 k, c. Since this frequency of 49 k. c. is that of the circuit 23', the point Z will have a direct current potential which is positive with respect to ground, due to rectification in the rectifier circuit including diode anode 22.

As a consequence the bias of the control grid of tube 30 will be positive thereby increasing the gain of tube 30, and increasing the magnitude of the control grid to cathode capacity of tube 30. An increase in this control grid to cathode capacity magnitude results in an increase inthe total capacity in the tank circuit 4; it therefore follows that the frequency of the tank circuit will be reduced. It will be understood that the constants of the oscillator frequency control device are so chosen that a change in frequency of the tank circuit away from the desired frequency value will. result in the production of control bias at point Z in the discriminator network suflicient in magnitude to vary the control grid to cathode capacity of tube 30 in that sense which will restore the tank circuit frequency to the desired value. In the illustrative case given above, the control grid to cathode capacity of tube 30 will be increased to such a'point'that the frequency of the tank circuit 4 will be decreased from 1951 k. c. to 1950 k. c. Those skilled in the art will readily'be able to work out the circuit constants for the reverse case wherein the tank circuit frequency shifts to a value below the desired value.

A practical difficulty of the receiving system shown in Fig. 1 is found in the difficulty of dislodging a. strong signal to make way for a weaker one on a closely adjacent channel, without having 4the weaker station jump right across the band to disappear on the other side. To overcome this difficulty the switch 29 is used; the function of the switch is to ground the bias voltage derived from point Z, and render the AFC system .inoperativel during tuning. After the signal-has been tuned in the switch 29 may be opened, and the ground connection removed. The manner of closing switch 29 during tuning depends upon theconstruction of the controls of thereceiver.- If there are too many operating controls on the panel of the receiver, it will be advisable to mechanically couple switch 29 with the tuning knob II so that the switch 29 will be automatically closed when the operators hand moves the tuning knob.

For example, the tuning knob may be depressible against a spring to close switch 29, and rotary motion of the tuning knob then instituted to-vary the position of the rotors of the tuning condensers. Any other mechanical arrangement can be employed to keep the switch 29 closed during tuning of the receiver. If there are not many controls on the receiver panel, the switch 29 may be operated as a separate switch to be used only before tuning from one station to another one.

In Fig. 2, the frequency control tube has its plate impedance vary With the bias upon the grid. When coupled to the tank circuit through condenser 69 (whose reactance is equal to Rp), the frequency change of the tank circuit, in kilocycles, for a given proportionate variation in Rp is independent of frequency, and is fixed by the time constant of Rp and the tank tuning capacity, If the latter is made as small as possible, for example of the order of 20 mmf., and the Rp of the tube is 10,000 ohms, a ten percent change in the latter will produce a frequency shift of 20 k. c.

In Fig. 3 is shown another AFC system modification; both the discriminator network and frequency control unit being modified and different from those disclosed up to this point. A portion of the I. F. carrier voltage at the input of the second detector is impressed upon the grid of tube T1. The receiver up to the second detector may be the same as that shown in Fig. 1. In the plate circuit of this tube the primaries P1 and Pz, of two similar I. F. transformers M4 and M5 are connected in parallel, and tuned to the exact center of the I. F. band; the resonance'curve of this composite primary is broadened by the shunt resistance R1 of from 25,000 to 50,000 ohms. The secondaries Si and Sz are tunedl respectively at equal increments above and below the I. F. band mid-frequency.

Owing to the presence of R1 across the common primary the latter is essentially a constant voltage source, and as there is no direct coupling between the two secondaries, the total effective coupling between the latter is negligible. The secondaries each operate into one of the anodes of double diode rectifier T2 having a common cathodewhich is ungrounded for direct current.

This connection gives a direct current output which is the difference between the rectified voltages from the two diodes. At a frequency near the resonance of S1 the output is negative, while at a symmetrical frequency near the resonance of S2 it is positive.

It is necessary to separate the resonant points of the two secondaries S1 and S2 by a minimum amount approximately equal to the I. F. midband frequency divided by the Q (erigir-mo) of the circuits. At 450 k. c., without considering losses introduced by the primary and by the diode load, a value of Q of 200 is about the highest that can be obtained in the usual size commercial I. F. coils. This corresponds to a minimum separation of 2.25 k. c. between the two secondary resonant points. After losses are accounted for, a separation of 4 or 5 k. c. would probably be excellent at this frequency. If a more selective discriminating device were necessary, recourse must be had to bulkier coils, or else a double heterodyne system used as in Fig. 1, the tube T1 operating as a converter with its output circuit tuned to a new and lower second I. F., 50 k. c. for example. The output from the balanced rectifier network is impressed upon the grid of the control tube T3. This tube, by means of any of several possible forms of reactive coupling circuit, causes the frequency of the oscillator to vary in accordance with the AFC bias, tending to return the I. F. carrier to the center of the I. F. band.

In the arrangement of Fig. 3 the coupling reactance is an inductance; this takes the form of a coil loosely coupled to the oscillator tank inductance. The excess inductive admittance of this coil is tuned out by a variable condenser across the control tube. The coupling becomes the familiar case of loosely coupled resonant circuits. It is well known that if the tuned winding is loosely coupled, as at Me, to the tank circuit 8| of the local oscillator the frequency of the latter may be considerably changed by varying the 'tuning of the coil 80, and, other things being equal, the amount of variation depends upon the damping of the circuit including coil 80. For the present purpose the coil 8011s mistuned by a fixed amount, and the damping is. varied by the Variable shunt tube resistance. This method was experimented with in the 4 to 10 megacycle band, and it was found that a frequency Variation of about one percent could be obtained over Athe whole band. It is pointed out that in this particular case one of the chief reasons for AFC would be to compensate for physical changes in the tank circuit due to thermal expansion, warpage and the like. The variable condensers in circuits 80, 8l are uni-controlled in order to keep the impedance reflected across the tank circuit a combination of a reactance and resistance of equal magnitude, or essentially so. This is the 45 condition, and results in no change in load on the tank circuit when the resistance is changed (electronically), and likewise maximum change in frequency. If the reactance and resistance. were very unequal, there would be a small change in frequency and a relatively large change in load. The uni-control is not intended to tune the circuits to the same resonance point; the control circuit is tuned so that it was always reactive, by an amount equal in this case to the average Rp of the tube.

' vary by +100 k. c.

In Fig. 4 there is shown an AFC system applied to a. receiver wherein is used the repeated application of the heterodyne principle, with the controlled oscillator operating at a fixed frequency, lower than that of the first tunable oscillator. Without the Afrequncy control feature, this is equivalent to a short wave converter connected to the input of a receiver intended for a lower frequency band; in the present case the second unit is provided with fixed input tuning. In the circuit a signal frequency of 50 megacycles has been taken asa concrete example, and a nominal first I. F. of 2 megacycles. has been assumed. It is further assumed that unwanted variations in the first oscillator may cause the actual I. F. to The coupling means between the output of the first converter and the input to the second is a band pass filter capable of passing 200 k. c. The second I. F. must be at least one half of this band width if image interference is tobe avoided. The second I. F. of 175 k. c. is demodulated. It is also used for frequency control.

Thus, the discriminator and frequency control operate on the second local oscillator. Frequency drifts as high as k. c. in either direction from 2175 k. c. will be corrected by the AFC system. Specifically, there may be employed in the AFC system of Fig. 4, the discriminator network of Fig. 3 followed by the frequncy control unit of Fig. 2. For the purpose of this divisional application, there is shown in Fig. 5 the manner of employing the discriminator of Fig. 3 with the control device of Fig. 2 in the AFCv system of Fig. 4. To presrve simplicity, the portion of the system of Fig. 4 preceding the first I. F. transformer is omitted.

This particular superheterodyne method is employed for the 50 megacycle signal range to provide a fixed nominal frequency for the oscillator to be controlled, it being simpler to control a nontunable oscillator than one that is tunable over a wide range coverage band. A frequency in the neighborhood of 2 megacycles is shown for this second controllable oscillator as this seems to be optimum to obtain a wide AFC variation in k. c. A very low frequency is not good because a large percent change would be required, and a very high frequency is subject to a limitation from tube capacities.

While I have indicated and described several systems for carrying my invention into effet, it will be'apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in they appended claims.

What is claimed is:

l. In a superheterodyne receiver of the type comprising a pair of frequency changer networks arranged in cascade and a local oscillator connected to each of said frequency changers, the local oscillator connected to thel second frequency changer being operated at a fixed frequency value and the local oscillator connected to the first frequency changer being. tunable over a desired frequency range, an automatic frequency control system including a discriminator network having an input circuit connected to receive the second frequency changer output energy and adapted to producea direct current voltage whose magnitude is dependent solely upon the frequency shift of the energy in the output of said second frequency changer from an assigned frequency value, and a frequency control device connected between the fixed frequency oscillator and the discriminator network, responsive to said direct current voltage, for adjusting the resonance of said fixedly tuned local oscillator in a direction such as to compensate for said shift.

2. In a superheterodyne receiver of the type provided with a first detector having a signal input circuit including means for tuning it over a wide range of signal frequencies, a local oscillator having a circuit with means for tuning it over a range of frequencies differing from the first range by a desired intermediate frequency, means for simultaneously adjusting both said tuning means to select different signals, an intermediate frequency transmission network, a second detector having an input circuit, fixedly tuned to said intermediate frequency, coupled to the transmission network a second local oscillator circuit, operatively associated with the second detector and being fiXedly tuned to a frequency differing from said intermediate frequency by a second intermediate frequency, a third detector circuit arranged to detect the second intermediate energy, and an automatic frequency control circuit for said fixedly tuned oscillator, said control circuit comprising an electron discharge device operatively associated with the second oscillator tank circuit to produce a reactance of predetermined sign and value thereacross, a discriminator network operatively associated with the second detector output circuit to derive a direct current voltage which depends in polarity and value upon the amount and direction of frequency shift of said second intermediate frequency from its fixed Value, and meanse for applying said direct current voltage to said discharge device in a sense to vary said reactance Value sufficiently to adjust the second oscillator tuning to compensate for said shift.

3. In a superheterodyne receiver of the type comprising a pair of frequency changer networks arranged in cascade and a local oscillator connected to each of said frequency changers, the local oscillator connected to the second frequency changer being operated at a fixed frequency value and the local oscillator connected to the first frequency changer being tunable over a desired frequency range, an automatic frequency control system including a discriminator network adapted to produce a direct current voltage whose magnitude is dependent solely upon the frequency shift of the energy in the output of said second frequency changer from an assigned frequency value, a frequency control device connected to the said fixed frequency oscillator, responsive to said direct current voltage, for adjusting the resonance of said fixedly tuned local oscillator in a predetermined direction, said discriminator network comprising a pair of diodes connected to rectify the second frequency changer output, each diode including a resistor in its space current path, and the voltages across the resistors being in polarity opposition to provide said direct current voltage.

4. In a superheterodyne receiver of the type comprising a pair of frequency changer networks arranged in cascade and a local oscillator connected to each of said frequency changers, the local oscillator connected to the second frequency changer being operated at a fixed frequency value and the local oscillator connected to the first frequency changer being tunable over a desired frequency range, an automatic frequency control system including a discriminator network adapted to produce a direct current voltage whose magnitude is dependent solely upon the frequency shift of the energy in the output of said second frequency changer from an assigned frequency value, and a frequency control device, responsive to said direct current voltage, for adjusting the resonance of said fixedly tuned local oscillator in a predetermined direction, said control device comprising a tube having its plate impedance connected across the tank circuit of the fixedly tuned oscillator, and means for impressing the direct current voltage upon a gain control electrode of the control tube.

5. In a superheterodyne receiver of the type comprising a pair of frequency changer networks arranged in cascade and a local oscillator connected to each of said frequency changers, the local oscillator connected to the second frequency changer being operated at a fixed frequency value and the local oscillator connected to the first frequency changer being tunable over a desired frequency range, an automatic frequency control system including a discriminator network having an .input circuit connected to receive the second frequency changer output energy and adapted to produce a direct current voltage whose magnitude is dependent solely upon the frequency shift of the energy in the output of said second frequency changer from an assigned frequency Value, a frequency control device connected between the fixed frequency oscillator and the discriminator network, responsive to said direct current voltage, for adjusting the resonance of said fixedly tuned local oscillator in a direction such as to compensate for said shift, said tunable oscillator being adjustable to frequencies of the order of 50 megacycles, and said other oscillator being tuned to operate at a frequency of the order of 2 megacycles.

6. In a superheterodyne receiver, adapted to receive signals in the 50 megacycle range, comprising a first detector network provided with a signal input circuit including means for tuning the latter over said range, a local oscillator network having a tank circuit with means for tuning the latter over a range of frequencies differing from the rst range by a desired operating intermediate frequency, means for simultaneously adjusting. both said tuning means, an intermediate frequency transmission network, a second detector network provided with an input circuit, which is fixedly tuned to said intermediate frequency, coupled to said transmission network, a second local oscillator network operatively associated with said second detector network and being fixedly tuned to a frequency differing from said intermediate frequency by a second operating intermediate frequency, the magnitude of said fixed oscillator frequency being of the order of 2 megacycles, means for detecting the second intermediate frequency energy, an automatic frequency control circuit for said fixedly tuned oscillator network, said control circuit comprising a discriminator network operatively associated with said second detector network to derive a direct current voltage therefrom depending in polarity and magnitude upon the direction and amount ofY frequency shift of said second intermediate frequency from its operating value, and means for applying said direct current voltage to said fixed frequency oscillator network in a sense to adjust the frequency of said tank circuit to compensate for said frequency shift.

7. The method of selective reception of signalmodulated carrier waves which includes: selectively tuning to any desired one of a plurality of modulated signal waves; generating and combining therewith a wave of approximately constant but incidentally variable frequency separation from the carrier frequency thereof to produce a modulated carrier wave of rst intermediate frequency; broadly selecting the carrier Wave of first intermediate frequency corresponding to said desired signal Wave to permit normal deviations in the frequency of said carrier Wave without substantial attenuation of the modulation frequencies thereof; generating and combining with said selected first intermediate-frequency carrier wave a second wave of substantially constant frequency to produce a modulated carrier Wave of second intermediate frequency; modifying the frequency of said second generated Wave within a range of a small per cent of its mean value in accordance with, and responsive to, predetermined deviations of the carrier frequency of said second intermediate-frequency wave from its normal frequency to restore it thereto; sharply selecting the carrier Wave of second intermediate frequency corresponding to said desired signal Wave; and thereafter converting and utilizing said modulated carrier Wave of second intermediate frequency.

8; A tunable radio receiver comprising; a'tunable first frequency changer for converting a selected signal Wave into a first modulated intermediate-frequency carrier wave of approximately constant frequency; broadly selective means for selecting said rst intermediate-frequency carrier wave While permitting normal deviations in the frequency thereof without substantial attenuation of the modulation frequencies thereof; an untunable second frequency changer for converting said selected rst intermediate-frequency carrier wave into a second modulated intermediate-frequency carrier Wave subject to minor deviations from a given normal frequency; means responsive to said deviations of said second intermediatefrequency carrier Wave for modifying the tuning of said second frequency changer to restore the frequency of said second intermediate-frequency carrier Wave to said given frequency; sharply selective means for selecting said second intermediate-frequency carrier Wave; and means for converting and utilizing said selected second modulated intermediate-frequency carrier Wave.

9. A tunable superheterodyne receiver comprising; a tunable frequency changer for converting a selected signal Wave into a rst modulated intermediate-frequency carrier Wave of approximately constant frequency; a first intermediatefrequency channel for broadly selecting said first intermediate-frequency carrier wave while permitting normal deviations in the frequency thereof without substantial attenuation of the modulation frequencies thereof; an untunable second frequency changer for converting said selected first intermediate-frequency carrier wave into a second modulated intermediate-frequency carrier Wave subject to minor deviations from a given normal frequency; a second intermediate-frequency channel for sharply selecting and amplifying said carrier waves of second intermediate frequency, said second channel having a center frequency equal to said normal frequency; a device responsive to deviations in frequency of said second intermediate-frequency carrier wave from said center frequency; and means controlled by said device for controlling the frequency of said untunable frequency changer to maintain the frequency of said second intermediate-frequency carrier Wave at said center frequency.

CHARLES TRAVIS. 

